The AC withstand voltage test is an indispensable part of the performance assessment of electrical equipment and insulating materials. Its core task is to verify the insulation strength and safety of the tested item under high voltage. Determining whether the tested item has been broken down is an extremely crucial step in the test process.
I. Basic Principles of the AC Withstand Voltage Test
The AC withstand voltage test mainly involves applying an AC voltage higher than the normal operating voltage to test the withstand capacity of the insulation system of the tested item. The test voltage is usually a certain multiple higher than the equipment’s rated voltage to simulate extreme electrical stress conditions. During the test, the voltage is gradually increased to the predetermined value, maintained for a certain period of time, and the insulation is observed to see if it can stably withstand. If the tested item experiences insulation failure during the test, i.e., a breakdown occurs, it indicates that its insulation performance is insufficient and there are potential safety hazards.

II. Physical Manifestations of Breakdown and Criteria for Judgment
Breakdown is the process where an insulating material develops an electrical conducting path either within or on its surface, resulting in a sharp increase in current. Specifically, it is manifested as:
Current surge?
During the withstand voltage test, the change in current is an important indicator for determining breakdown. Under normal circumstances, the test current remains at an extremely low level, showing a leakage current at the microampere level. If the test sample breaks down, the current will rapidly increase, possibly reaching milliampere or even higher levels. This sudden increase in current is usually accompanied by the activation of the equipment protection device or the automatic power-off of the test power supply.
2. Abnormal voltage waveform
During breakdown, the voltage waveform will exhibit significant distortion or sudden changes, manifesting as a sudden drop in voltage that fails to maintain the set test voltage. The moment of the voltage drop is often the direct evidence of breakdown occurrence.
3. Sound and odor cues
In some cases, breakdown may be accompanied by crackling sounds or arc sounds produced by local discharge, and even a burnt smell. These sensory signals provide auxiliary information for on-site personnel to make judgments.

III. Technical Means for Judging Breakdown
Current monitoring system
High-precision current sensors are the core equipment for detecting breakdown. By monitoring the test current in real time and comparing it with the preset threshold, it can determine whether the current is abnormal. Modern testing instruments are equipped with automatic alarm and power-off functions, ensuring that the power supply is quickly cut off in case of breakdown to prevent the accident from escalating.
2. Insulation Resistance Measurement
Although insulation resistance measurement is usually conducted before and after the withstand voltage test, its results can assist in judging the insulation condition. Breakdown will cause a significant drop in insulation resistance. If the resistance drops sharply during the test, one should be vigilant of the risk of breakdown.
3. Partial Discharge Detection
Partial discharge is an early sign of insulation defects. Using a partial discharge detector can capture the weak discharge activities. If the partial discharge amount increases sharply during the test, it may indicate that a breakdown is about to occur.

IV. Precautions during the Experiment Operation
The accurate determination of the AC withstand voltage test depends on proper operation and reasonable parameter setting. The test personnel should reasonably select the test voltage and duration based on the specifications of the tested item and the usage environment. The rising speed should be moderate to avoid misjudgment due to sudden voltage changes. Protective facilities should be provided at the test site to ensure the safety of personnel.
Furthermore, the recording and analysis of test data cannot be overlooked. Through detailed analysis of the current and voltage waveforms, potential insulation defects can be identified, and maintenance measures can be taken in advance to reduce the risk of equipment failure.

V. Case Study Analysis
During a high-voltage cable withstand voltage test, the current suddenly jumped from the microampere level to several tens of milliamperes, accompanied by a significant drop in the voltage waveform. The on-site operator immediately cut off the power supply. Subsequent inspection revealed that there was local mechanical damage to the cable insulation layer, which led to the breakdown. This case demonstrates that real-time monitoring of current and voltage is crucial for timely detection of breakdown incidents.

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